FIG. 1 shows a simplified block diagram of the GSM system (Global System for Mobile communications). Mobile station MS is connected over a radio path with some base transceiver station BTS, in the case shown in FIG. 1 with base transceiver station BTS1. Base Station Sub-system BSS includes Base Station Controller BSC and subordinated base transceiver stations BTS. Several base station controllers BSC are usually subordinated to Mobile Services Switching Centre MSC. Mobile services switching centre MSC is in connection with other mobile services switching centres, and through Gateway Mobile Services Switching Centre GMSC the GSM network is connected with other networks, such as public switched telephone network PSTN, another public land mobile network PLMN, ISDN network or intelligent network IN. The operation of the whole system is monitored by Operation and Maintenance Centre OMC. The subscriber information of mobile station. MS is permanently stored in the system's Home Location Register HLR and it is temporarily stored in that Visitor Location Register VLR, in the area of which mobile station MS is located at each time. Location information of the mobile station is stored in visitor location register VLR with Location Area LA precision. The geographical area monitored by the visitor location register is divided into one or more location areas. One or more base transceiver stations BTS may operate in each location area. Each base transceiver station BTS is constantly transmitting such information on its broadcast channel which is intended for all mobile stations MS and which contains Base Station Identity Code BSIC, location area identifier LAI and information about such frequencies of adjacent base transceiver stations which mobile station MS should measure.
In mobile communications systems of the cellular type, radio coverage is implemented with several radio cells which are located with some mutual overlapping. The mobile station may move freely in the area of the mobile station network and may connect with the best base transceiver station signal available at each time. When the mobile station moves from one cell to another during a call, handover to a new radio cell is performed based on predetermined handover criteria. Handover should be carried out in a way causing the least possible interference with the call going on. To make it possible in a mobile communications system to find out the need for handover and to choose a suitable target cell for the handover, various measurements of connection quality are needed as well as measurements of the field strengths of adjacent cells. Handover is normally caused due to criteria of the radio path, but it can be done also for other reasons, such as e.g. in order to divide the load or to reduce transmission powers. Handover may also be carried out inside the cell from one traffic channel to another.
Mobile station MS is constantly measuring the broadcast channel signals of those base transceiver stations BTS which are located closest to itself, e.g. in order to determine the base transceiver station providing the best signal and to prepare for a possible exchange of cell. Based on the adjacent cell information transmitted by each base transceiver station on its broadcast channel, the mobile station will identify those adjacent cells which it should watch. Mobile station MS regularly transmits the measurement results as a report message through serving base transceiver station BTS1 to base station controller BSC. The report message contains the measurement results of the serving base transceiver station and of no more than the six best adjacent base transceiver stations. Base transceiver station BTS performs signal level and quality measurements of all radio connections going on at the base transceiver station.
Handover from a serving cell to some neighboring cell or to another channel of the serving cell may take place e.g. when measurement results of the mobile station and/or the base transceiver station indicate a low signal level and/or quality in the current channel of the serving cell and a better signal level is available from an ambient cell or a better signal quality can be achieved on another channel, or when some ambient cell/other channel allows communication at lower transmission power levels. The choice of target cell for the handover is influenced e.g. by the signal level and/or load of the target cell. Generally used handover criteria are thus e.g. the signal level and quality of the radio connection, the signal levels of the initial cell and the target cell, the signal level of the initial cell, the transmission power which is demanded of the mobile station and which is permissible for the mobile station in the target cell. Handover from one traffic channel to another is usually carried out when handover criteria established by the operator are fulfilled. It may be necessary to perform handover also due to excessive loading.
Base station controller BSC makes the decisions relating to handover. If the target cell is subordinated to another base station controller BSC, the handover may be done under the control of mobile services switching centre MSC. Handover decisions can also always be made centrally in mobile services switching centre MSC. When required, base station controller BSC gives a handover command through the base transceiver station to mobile station MS.
FIG. 2a illustrates signalling relayed in handover between transceiver units TRX of two base transceiver stations in the GSM network. In the start-up situation of the handover in FIG. 2a, a radio communication has been set up for mobile station MS, e.g. in order to transmit speech, data or signalling, by way of initial TRX. When base station controller BSC finds that there is a need for handover to a channel of another base transceiver station, base station controller BSC will transmit to target-TRX Channel activation message 21, notifying of the need for handover and requesting target-TRX to assign a radio channel for the mobile station. The target TRX assigns a radio channel for communication and notifies base station controller BSC of this in Channel activation ack message 22. Base station controller BSC transmits the new radio channel information to the initial-TRX of the base transceiver station now to be used in a Handover command (message 23). The base transceiver station transmits the radio channel information to mobile station MS in a Handover command message 24. Mobile station MS carries out the handover by moving over to the new indicated radio channel and transmits to target-TRX a connection request in Handover access burst message 25. Mobile station MS transmits these Handover access burst messages at certain intervals, until it receives a reply from the base transceiver station. Having received the connection request of mobile station MS, the target-TRX transmits to the mobile station an acknowledgement in Physical info message 31, also stating e.g. the timing advance needed by the mobile station for its transmission. Then a link connection is set up between mobile station MS and target-TRX of the new base transceiver station by messages 32 SABM and 33 UA. After the link connection has been set up, mobile station MS transmits to target-TRX an acknowledgement of the successful performance of the handover in Handover complete message 34. The information about successful handover is relayed to base station controller BSC in Handover complete message 35. Mobile station MS continues its communication via the target-TRX. The handover initial-TRX and target-TRX may also be located in the same base transceiver station. If the initial-TRX and the target-TRX are located subordinated to different base station controllers, the signalling will be relayed through mobile services switching centre MSC.
Handover will fail at times due to interference on the radio path. FIG. 2b illustrates signalling in a situation where handover has failed. As regards messages 21-25, the handover begins in the same way as was described earlier in connection with FIG. 2a. However, if mobile station MS receives no reply to its connection requests (messages 25) within a certain time established in advance, after the base transceiver station has failed to receive the connection requests or after the mobile station has failed to receive the base transceiver station's acknowledgement of the connection requests, mobile station MS will return to that channel of the initial-TRX, from which the handover started. A link connection is set up between mobile station MS and the initial-TRX by messages 36 SABM and 37 UA. After the link connection has been set up, mobile station MS transmits to the initial-TRX a message of the failure of the handover in a Handover failure message 38. Information about the handover failure is relayed to base station controller BSC in Handover failure message 39. Mobile station MS continues its communication via the initial-TRX.
FIG. 2c shows a signalling diagram of handover during a call in handover within a cell from an initial channel to a target channel. When base station controller BSC finds that there is a need for handover, it transmits to the target channel a channel assignment request in a Channel activation message 21 in the same way as was described earlier in connection with FIG. 2a. Also the acknowledgement and the notification of radio channel data in message 22 are transmitted in the way described in connection with FIG. 2a. Base station controller BSC transmits to the initial channel now to be used for communication new radio channel data in Assignment command message 23, which is relayed further to mobile station MS in message 24. After mobile station MS has received the assignment command to a new channel, it transmits on target channel a message 32 to set up a SABM link connection. The base transceiver station acknowledges this with message 33 UA. To complete the handover, the mobile station transmits on the new channel a message of the successful channel assignment in Assignment complete message 34. The base transceiver station relays this information further to the base station controller in Assignment complete message 35.
FIG. 2d is a signalling diagram of a failed handover within a cell during a call. Messages 21-24 and 32 are transmitted in the same way as has been described above in connection with FIG. 2c. The handover within the cell fails after the base transceiver station has failed to receive the link connection set up message or after the mobile station has failed to receive the base transceiver station's acknowledgement of the set up of the link connection. Having waited for the reply for a certain time, mobile station MS transmits to the network a message of the failed channel assignment in Assignment failure message 38 on that initial channel from which handover began. The base transceiver station relays the message of the failed channel assignment to base station controller BSC in Assignment failure message 39.
In a mobile communications system implemented with Code Division Multiple Access (CDMA) technology, such handover which is carried out in the ways described above is called hard handover. In addition, CDMA systems use so-called soft handover, where in the course of handover the mobile station may be simultaneously connected with the network through several base transceiver stations. When the signal of some station of these base transceiver stations shows this station to be the best, the mobile station's connections with other base transceiver stations are cut off and the call is continued only through this one base transceiver station which is best. Soft handover prevents handover back and forth between base transceiver stations, when the mobile station is located in a marginal area of the cells.
In cellular radio systems of two different frequency ranges, handover from one frequency range to another is a bit more problematic, especially so when there is only one broadcast channel in the cellular radio system. E.g. in a combination of a GSM system operating in a frequency band of approximately 900 MHz and a DCS system operating in a frequency band of approximately 1800-1900 MHz one broadcast channel can be implemented e.g. in a GSM frequency range. Hereby mobile station MS is able to measure only the base transceiver station signals of the transmission at GSM frequency. Hereby the call set up is always begun from the frequency range of the broadcast signal, because the broadcast is needed for synchronising the mobile station into the network, and during the call handover is attempted to another frequency range. It is known in the case of one broadcast channel to perform handover between GSM and DCS frequencies in two different ways: 1) by setting in the handover criterion a sufficiently high threshold value, which the broadcast signal must meet before handover will be carried out to another frequency range, or 2) by putting in test use a traffic channel of another frequency range, whereby normal uplink and downlink measurements are made on the traffic channel during the call and a new handover is performed back to the first frequency range, if the signal is not good enough. In the second method presented above, the criterion for performing handover to another frequency range may be set as a lower value than in the first method, e.g. as an average value.
It is a problem in state-of-the-art methods of implementing handover, and especially in a cellular radio system of two frequency ranges presented above, in the first method that a safe handover threshold value is uneconomic from the viewpoint of network capacity, whereby it wastes network capacity. Since the range of a base transceiver station signal of a larger frequency range is smaller than the range of a base transceiver station signal of a smaller frequency range and signals of different frequencies will change at a different rate in relation to the distance, the safe handover threshold value must be made even too demanding e.g. as regards handover taking place near the base transceiver station. In addition, such a safe threshold value will prevent full utilisation of another frequency range in the communication. In the second method of implementation presented above, it is a problem that the connection quality will deteriorate in the other frequency range while measurements are performed on the traffic channel, if the radio signals are not good enough after all and a new handover must be performed in order to move the connection back to the first frequency range. The quality of the service will then suffer, as the mobile station is exposed to the poor radio signal.